This invention relates to an acidic viscoelastic surfactant fluid based on a cationic surfactant. More particularly it relates to such a fluid that has low viscosity as formulated, undergoes an increase in viscosity as the acid is consumed, and returns to approximately the initial viscosity as the last of the acid is consumed. Most particularly it relates to the use of this fluid as a diverting fluid or as the main treatment fluid in oilfield treatments such as matrix acidizing and acid fracturing.
In matrix acidizing treatments of carbonate reservoirs the placement of the stimulation fluid is critical to success. Self-diverting stimulation fluid systems based on viscoelastic surfactants (called “viscoelastic diverting acid” or VDA) have been described; see U.S. Pat. No. 6,399,546 and U.S. patent application Publication No. 2003-0119680, both of which are assigned to the same assignee as the present invention and both of which are hereby incorporated in their entirety. That type of system, based for example on betaine surfactants, will be called the “current” system here. The current VDA system has a low viscosity at low pH (strong acid) and undergoes an in-situ increase in viscosity when the acid is being spent by reaction with calcium carbonate (or other reactive minerals) in the formation. Not to be limited by theory, but the chemical mechanism is believed to be the transformation from spherical or short worm-like surfactant micelles to highly elongated surfactant micelles. The spent fluid is chemically stable up to about 300° F. (149° C.); breaking of the viscous fluid after the treatment (reduction of viscosity rather than chemical destruction of the surfactant) is caused by contact with crude oil or condensates in the reservoir or by dilution by formation water. The use of mutual solvents as pre and/or post flushes has also been applied to assist in the breaking, but it is difficult to achieve effective contact of the VDA with the mutual solvent within a formation.
Although the current VDA has been applied in dry reservoirs and has been found to be successful in some cases, concern remains as to whether spent VDA can be cleaned up effectively with formation fluid or with flushes. It would therefore be highly desirable to have a fluid system that self breaks in the formation, which will greatly simplify job design and execution. It would also be desirable to have an acidic viscous fluid system, such as a VDA fluid system, that is formulated from cationic surfactants as opposed to the zwitterionic surfactants of the current VDA because the salt concentration generated through acid spending is high enough to break viscous fluids made from cationic surfactants, which are more sensitive to high salt concentrations than viscous fluids made from the zwitterionic surfactants used for the current VDA.